Methods as well as devices for the adjustment of states of charge of battery cells that are operated electrically in parallel connection are known in basic principle, so that a separate documentation of publications for this is not needed. Such battery cells or batteries are often employed in high-voltage battery systems, such as are used, for example, in electrically drivable motor vehicles or the like. In the case of battery cells that are electrically connected in series, it is known to carry out a charge equilibration with respect to the battery cells in order to reduce thereby different states of charge of the battery cells connected in series. This operation is also referred to as balancing. For this purpose, in the case of battery cells that are electrically connected in series, a respective switchable electric resistor is connected in parallel to each individual battery cell. These resistors can be activated selectively in order to be able to adjust the states of charge of the individual battery cells. For this purpose, the respective states of charge of the battery cells are recorded separately and the resistors are correspondingly activated.
For battery cells that are electrically connected in parallel, the aforementioned method cannot be used. In order to be able to achieve a charge adjustment between individual battery cells in this case, it is necessary to disconnect the particular battery cell from the assembly by switching technology and to charge or discharge it to a predetermined state of charge by means of a charging device or a discharging device. Such a method is very complex. For this reason, a balancing is not in very widespread use for battery cells connected in parallel.
A unique feature of battery cells that are electrically connected in parallel consists in the fact that all of the battery cells that are electrically connected in parallel have essentially the same electrical voltage. If, in the case of individual battery cells, even slight differences in voltage exist in relation to their respective individual no-load voltage, then, at the instant of interconnection to create a parallel circuit, a corresponding compensating current flows. The compensating current is limited via the particular internal resistances of the battery cells in question as well as by way of contact resistances between connection terminal contacts of the battery cells and an electrical connection for producing the parallel connection of the battery cells. Generally, the internal resistances of the battery cells are very small and often lie in the low million range or even below it. This effect is enhanced due to the fact that the different battery cells that are electrically connected in parallel generally have slight differences in voltage in relation to the no-load voltage. For this reason, for an identical structural design, the respective states of charge of the battery cells deviate from one another. As a result of this, it is possible to further enhance the aforementioned effect.
A battery cell is a device that has two electrodes that interact electrochemically with each other. The interaction can occur with supplemental assisting action of an electrolyte. Battery cells, which are also referred to as galvanic cells, are preferably reversible in terms of their function, such as when they are utilized, for example, for batteries in the form of rechargeable batteries. On account of the electrochemical interaction of the electrodes, a direct current voltage that is specific to the battery cell chemistry is established at the electrodes and is supplied by way of connection terminal contacts of the respective battery cell that are connected to the electrodes.
Inside the battery, the individual battery cells are often connected to one another in an electrically conductive manner by means of bus bars or similar electrical conductors in the desired type of circuit, in order to be able to supply a desired direct current voltage at the connection terminal poles of the battery together with a likewise desired ability to bear loads. Such batteries are employed, for example, as lead acid batteries in the motor vehicle sector, as nickel cadmium batteries in the aircraft sector, and, for interruption-free current supplies, as lithium ion batteries in the field of small household appliances and/or the like, but recently also in electrically driven vehicles.
Electrically drivable motor vehicles comprise, as a rule, an electrical system with a battery as well as a drive device connected to the electrical system. Such motor vehicles are, for example, electric vehicles, hybrid vehicles, in which a drive is possible both by means of an electrical drive device and also by means of an internal combustion engine, or the like. In such motor vehicles, the battery is often designed as a high-voltage battery system.
When a high-voltage battery system of a motor vehicle is utilized, energy losses arise on account of the above-described compensating currents and can be detrimental to the range of travel of the motor vehicle. More efficient battery cells are subject to greater load in regard to the battery cells, for which reason they are subject to enhanced aging. Here, too, the operational readiness of battery cells, which are electrically connected in parallel and are part of the battery, is overall reduced. In the process, it has proven to be especially detrimental that an energy balancing for producing a uniform state of charge between the respective battery cells is not possible owing to the parallel connection. For a battery having such a parallel connection of battery cells, there results a greater power loss, the overall further consequence of which can be a premature aging of the battery. Furthermore, there exists the danger of overloading battery cells of weaker power.